defective

Grammar. (of an inflected word or its inflection) lacking one or more of the inflected forms proper to most words of the same class in the language, as English must, which occurs only in the present tense.

mid-14c., from Middle French défectif (14c.) and directly from Late Latin defectivus, from defect-, past participle stem of deficere (see deficient). A euphemism for "mentally ill" from 1898 to c.1935. Related: Defectively; defectiveness.